MapKit directions and current location

If I create a route of 2 segments, A to B and B to C, along with a current location of D, how can I determine which segment "contains" the location point D? I would like to compute the time to the final destination but need to know which segment I am currently following. Hope this make sense. Thanks for any suggestions. NOTE: I do NOT want to simply compute a new route from D to C as that would not necessarily follow the routes already defined.